GaN-based light emitting diodes (LEDs) have been applied and developed for about 10 years. The GaN-based LEDs have significantly advance the LED technologies and has been employed in various applications including full color LED displays, LED traffic signals, white LEDs and the like.
Recently, high-efficiency white LEDs have been expected to replace fluorescent lamps. In particular, the efficiency of white LEDs is approaching that of ordinary fluorescent lamps.
Two major approaches to improve the efficiency of LEDs have been attempted. The first approach is to enhance the internal quantum efficiency determined by the crystal quality and the epitaxial layer structure, and the second approach is to increase the light extraction efficiency.
Since the internal quantum efficiency currently reaches 70-80%, there is little room for further improvement of the internal quantum efficiency. However, the light extraction efficiency may be further improved. In the light extraction efficiency, it is most important to eliminate internal loss of light due to total internal reflection.
A light emitting diode with improved light extraction efficiency through the prevention of total internal reflection has been disclosed in Applied Physics Letters, Vol. 84, No. 6, pp. 855-857, entitled “Increase in the extraction efficiency of GaN-Based light emitting diodes via surface roughening” by Fujii et al.
The LED is formed by depositing an N-type semiconductor layer, an active layer and a P-type semiconductor layer on a sapphire substrate, bonding the semiconductor layers on a submount, separating the semiconductor layers from the substrate using a laser lift-off (LLO) technique, and then roughening a surface of the N-type semiconductor layer. As the surface of the N-type semiconductor layer is roughened, the extraction efficiency of light emitted to the outside through the N-type semiconductor layer can be improved.
However, since there is a limitation in improvement of the light extraction efficiency, a chip area should be increased in order to obtain necessary light output per unit chip. The increase in the chip area results in the increase in manufacturing costs per chip. Accordingly, a new LED capable of increasing light output from the same unit chip area is required.
Meanwhile, an LED is repeatedly turned on/off according to the direction of a current from an AC power supply. Thus, in a case where the LED is used while connecting directly to the AC power supply, there is a problem in that the LED does not continuously emit light and may be easily damaged due to reverse current.
To solve such a problem of the light emitting diode, a light emitting diode that can be used while connected directly to a high-voltage AC power supply has been disclosed in PCT Publication No. WO 2004/023568 A1 entitled “Light-Emitting Device Having Light-Emitting Elements” by Sakai et al.
According to PCT Publication No. WO 2004/023568 A1, light emitting diodes (light emitting cells) are two-dimensionally connected in series on an insulating substrate such as a sapphire substrate to form LED arrays. Two LED arrays are connected in reverse parallel on the sapphire substrate. As a result, a single chip light emitting device that can be driven by means of an AC power supply is provided.
In such a light emitting device, since LED arrays are alternately operated under an AC power supply, its light output is considerably limited as compared with a case where light emitting cells are simultaneously operated. Thus, it is further necessary to improve the light output per unit area within such a light emitting device.
An object of the present invention is to provide a light emitting device capable of increasing light output per unit area.
Other object of the present invention is to provide a light emitting device with improved light output which can be driven under an AC power supply.
To achieve the aforementioned objects of the present invention, the present invention provides a light emitting device having vertically stacked light emitting diodes. A light emitting device according to an aspect of the present invention comprises a lower semiconductor layer of a first conductive type positioned on a substrate. A lower semiconductor layer of a second conductive type is positioned on the lower semiconductor layer of a first conductive type, and a upper semiconductor layer of a second conductive type is positioned on the lower semiconductor layer of a second conductive type. Further, an upper semiconductor layer of a first conductive type is positioned on the upper semiconductor layer of a second conductive type. Furthermore, a lower active layer is interposed between the lower semiconductor layer of a first conductive type and the lower semiconductor layer of a second conductive type, and an upper active layer is interposed between the upper semiconductor layer of a second conductive type and the upper semiconductor layer of a first conductive type. In addition, a separating layer is interposed between the lower semiconductor layer of a second conductive type and the upper semiconductor layer of a second conductive type. Accordingly, there is provided a light emitting device having a structure in which a lower light emitting diode comprising the lower semiconductor layer of a first conductive type, the lower active layer and the lower semiconductor layer of a second conductive type and an upper light emitting diode comprising the upper semiconductor layer of a second conductive type, the upper active layer and the upper semiconductor layer of a first conductive type are separated by the separating layer.
Here, the separating layer is a layer for electrically separating the lower and upper light emitting diodes, and may be, for example, an insulating layer or high-resistance semi-insulating layer. Further, the first and second conductive types indicate N-type and P-type, or P-type and N-type, respectively.
Meanwhile, first cladding layers may be arranged such that the lower active layer is interposed between the first cladding layers, and second cladding layers may be arranged such that the upper active layer is interposed between the second cladding layers.
The lower semiconductor layer of a second conductive type may be positioned on one region of the lower semiconductor layer of a first conductive type, the upper semiconductor layer of a second conductive type may be positioned on one region of the lower semiconductor layer of a second conductive type, and the upper semiconductor layer of a first conductive type may be positioned on one region of the upper semiconductor layer of a second conductive type. At this time, a first-type lower electrode may be formed on other region of the lower semiconductor layer of a first conductive type, and a second-type lower electrode may be formed on other region of the lower semiconductor layer of a second conductive type. Further, a second-type upper electrode may be formed on other region of the upper semiconductor layer of a second conductive type, and a first-type upper electrode may be formed on the upper semiconductor layer of a first conductive type. The second-type lower and upper electrodes are connected to one terminal of an external power supply and the first-type lower and upper electrodes are connected to the other terminal thereof, so that the light emitting device can be driven. Further, two external power supplies are connected to the first-type and second-type lower electrodes and the second-type and first-type upper electrodes, respectively, so that the upper and lower light emitting diodes can be individually driven.
Meanwhile, a codoped layer may be interposed between the lower semiconductor layer of a first conductive type and the lower active layer or between the upper semiconductor layer of a second conductive type and the upper active layer.
A light emitting device according to the aspect of the present invention may comprise a plurality of light emitting cells positioned on a substrate. Each of the light emitting cells comprises a lower semiconductor layer of a first conductive type positioned on the substrate. A lower semiconductor layer of a second conductive type is positioned on one region of the lower semiconductor layer of a first conductive type, and an upper semiconductor layer of a second conductive type is positioned on one region of the lower semiconductor layer of a second conductive type. Further, an upper semiconductor layer of a first conductive type is positioned on one region of the upper semiconductor layer of a second conductive type. Furthermore, a lower active layer is interposed between the lower semiconductor layer of a first conductive type and the lower semiconductor layer of a second conductive type, and an upper active layer is interposed between the upper semiconductor layer of a second conductive type and the upper semiconductor layer of a first conductive type. In addition, a separating layer is interposed between the lower semiconductor layer of a second conductive type and the upper semiconductor layer of a second conductive type. Accordingly, there is provided a plurality of light emitting cells each having vertically stacked lower and upper light emitting diodes, wherein the lower light emitting diode comprises the lower semiconductor layer of a first conductive type, the lower active layer and the lower semiconductor layer of a second conductive type and the upper light emitting diode comprises the upper semiconductor layer of a second conductive type, the upper active layer and the upper semiconductor layer of a first conductive type.
Each of the light emitting cells may have first cladding layers between which the lower active layer is interposed and second cladding layers between which the upper active layer is interposed.
Meanwhile, each of the light emitting cells may further comprise a first-type lower electrode formed on other region of the lower semiconductor layer of a first conductive type. Further, a second-type lower electrode may be formed on other region of the lower semiconductor layer of a second conductive type, a second-type upper electrode may be formed on other region of the upper semiconductor layer of a second conductive type, and a first-type upper electrode may be formed on the upper semiconductor layer of a first conductive type. The first-type lower and upper electrodes of each light emitting cell may be electrically connected to the second-type lower and upper electrodes of an adjacent light emitting cell, respectively, to provide an array of light emitting cells connected in series. According to this embodiment, there can be provided an upper array in which the upper light emitting diodes are connected in series with one another and a lower array in which the lower light emitting diodes are connected in series with one another.
In the meantime, in each of the light emitting cells, a codoped layer may be interposed between the lower semiconductor layer of a first conductive type and the lower active layer or between the lower semiconductor layer of a second conductive type and the upper active layer.
A light emitting device according to another aspect of the present invention comprises a lower semiconductor layer of a first conductive type positioned on a substrate. A semiconductor layer of a second conductive type is positioned on the lower semiconductor layer of a first conductive type, and an upper semiconductor layer of a first conductive type is positioned on the semiconductor layer of a second conductive type. Furthermore, a lower active layer is interposed between the lower semiconductor layer of a first conductive type and the semiconductor layer of a second conductive type, and an upper active layer is interposed between the semiconductor layer of a second conductive type and the upper semiconductor layer of a first conductive type. Accordingly, there is provided a light emitting device in which a lower light emitting diode comprising the lower semiconductor layer of a first conductive type, the lower active layer and the semiconductor layer of a second conductive type and an upper light emitting diode comprising the semiconductor layer of a second conductive type, the upper active layer and the upper semiconductor layer of a first conductive type are vertically stacked. Thus, light output per unit area of the light emitting device is increased as compared with a conventional light emitting device. Further, when the lower and upper active layers are formed of different materials, a light emitting device capable of emitting light with various wavelengths can be provided.
Meanwhile, first cladding layers may be arranged such that the lower active layer is interposed between the first cladding layers, and second cladding layers may be arranged such that the upper active layer is interposed between the second cladding layers.
In the meantime, the semiconductor layer of a second conductive type may be positioned on one region of the lower semiconductor layer of a first conductive type, and the upper semiconductor layer of a first conductive type may be positioned on one region of the semiconductor layer of a second conductive type. At this time, a first-type lower electrode may be formed on other region of the lower semiconductor layer of a first conductive type, a second-type electrode may be formed on other region of the semiconductor layer of a second conductive type, and a first-type upper electrode may be formed on the upper semiconductor layer of a first conductive type. Thus, the second-type electrode is connected to one terminal of an external power supply and the first lower and upper electrodes are connected to the other terminal thereof, so that the light emitting device can be driven through a single external power supply. Further, the lower and upper light emitting diodes of the light emitting device may be individually driven using two external power supplies.
Furthermore, a codoped layer may be interposed between the lower semiconductor layer of a first conductive type and the lower active layer or between the semiconductor layer of a second conductive type and the upper active layer. The codoped layer is a semiconductor layer doped together with N-type and P-type ions, and may be, for example, GaN doped together with Mg and Si or with Mg and In. Particularly, the codoped layer is formed on a P-type semiconductor layer to enhance the crystal quality of an active layer formed thereon.
A light emitting device according to the another aspect of the present invention may comprise a plurality of light emitting cells positioned on a substrate. Each of the light emitting cells comprises a lower semiconductor layer of a first conductive type positioned on the substrate. A semiconductor layer of a second conductive type is positioned on one region of the semiconductor layer of a second conductive type, and an upper semiconductor layer of a first conductive type is positioned on one region of the semiconductor layer of a second conductive type. Furthermore, a lower active layer is interposed between the lower semiconductor layer of a first conductive type and the semiconductor layer of a second conductive type, and an upper active layer is interposed between the semiconductor layer of a second conductive type and the upper semiconductor layer of a first conductive type. Accordingly, there is provided a plurality of light emitting cells each having vertically stacked lower and upper light emitting diodes, wherein the lower light emitting diode comprises the lower semiconductor layer of a first conductive type, the lower active layer and the semiconductor layer of a second conductive type, and the upper light emitting diode comprises the semiconductor layer of a second conductive type, the upper active layer and the upper semiconductor layer of a first conductive type. Thus, there can be provided a light emitting device in which the light emitting cells are electrically connected to allow the device to be driven under an AC power supply, and light output per a unit area of the light emitting device is increased as compared with a conventional light emitting device. Further, when the lower and upper active layers are formed of different materials, a light emitting device in which the lower and upper light emitting diodes emit light with different wavelengths can be provided.
Each of the light emitting cells may have first cladding layers between which the lower active layer is interposed and second cladding layers between which the upper active layer is interposed.
Meanwhile, each of the light emitting cells may further comprise a first-type lower electrode formed on other region of the lower semiconductor layer of a first conductive type. Further, a second-type electrode may be formed on other region of the semiconductor layer of a second conductive type and a first-type upper electrode may be formed on the upper semiconductor layer of a first conductive type. The first-type lower and upper electrodes of each light emitting cell may be electrically connected to the second-type electrode of an adjacent light emitting cell. Thus, the second-type electrode of each light emitting cell is electrically connected to the first-type lower and upper electrodes of another adjacent light emitting cell. Accordingly, there is provided an array in which the light emitting cells are connected in series with each other.
In some embodiments of the present invention, other second-type electrode may be formed on another region of the semiconductor layer of a second conductive type, in addition to the first-type lower electrode, the second-type electrode and the first-type upper electrode. The first-type lower electrode of each light emitting cell is connected to the second-type electrode of an adjacent light emitting cell, and the other second-type electrode of each light emitting cell is connected to the first-type upper electrode of the adjacent light emitting cell. Further, the second electrode of each light emitting cell is connected to the first-type lower electrode of other adjacent emitting cell, and the first-type upper electrode of each light emitting cell is connected to the other second electrode of the other adjacent light emitting cell. According to these embodiments, there can be provided an array in which the upper light emitting diodes of the light emitting cells are connected in series and an array in which the lower light emitting diodes of the light emitting cells are connected in series, and the two arrays are then connected in reverse parallel to each other. Further, nodes between the light emitting diodes in one array are connected to nodes between the light emitting diodes in the other array, respectively, so that an electrically stable operation can be obtained.
Meanwhile, in each of the light emitting cells, a codoped layer may be interposed between the lower semiconductor layer of a first conductive type and the lower active layer or between the semiconductor layer of a second conductive type and the upper active layer.
According to the embodiments of the present invention, there is provided a light emitting diode capable of improving light output per unit area using vertically stacked light emitting diodes. Further, there is provided a light emitting device which emits light with a plurality of wavelengths by forming active layers of the vertically stacked light emitting diodes with different materials. Furthermore, there is provided a light emitting device with improved light output which can be driven under an AC power supply by electrically connecting a plurality of light emitting cells having vertically stacked light emitting diodes.